Spent caustic liquors are generated in chemicals and petrochemicals manufacturing operations and are characterized by high pH (about 13.0). The spent caustic liquors contain organic substances and strong bases, usually sodium hydroxide, and often contain sulfides. The caustic wastes can be treated effectively by wet oxidation for almost complete destruction of the pollutants, whereby sulfides are oxidized to sulfates and the organic substances are primarily converted to carbon dioxide and water.
Such wet oxidation processes are shown for example in Schoeffel U.S. Pat. No. 3,097,988 and Pradt U.S. Pat. No. 3,714,911, which disclose methods of treating alkaline pulping black liquors by wet oxidation.
A summary of the state of the art has been presented by A. K. Chowdhury and A. R. Wilhelmi in a paper entitled "Treatment of Spent Caustic Liquors by Wet Oxidation", presented at the 8th Annual Industrial Pollution Conference sponsored by Water and Wastewater Equipment Manufacturers Association, June 3-6, 1980, Houston, Texas.
The wet oxidation of the caustic wastes is usually carried out in the temperature range of 350.degree.-650.degree. F., and at a pressure in the range of 300-3500 psi. To achieve the desired reaction temperature, particularly at the higher end of the temperature range, the feed often requires preheating almost up to the reaction temperature. Feed preheating is usually achieved by exchanging heat between the feed and the effluent in a heat exchanger followed by supplemental heating by a "hot oil" heater.
The high preheat and reaction temperatures, often necessary for the treatment of caustic wastes, require special materials of construction of the wet oxidation system components to prevent corrosion. High nickel alloys such as Incoloy 800 are resistant to alkali at higher temperatures but are expensive. Also, when the wet oxidation system components are made of Incoloy 800, preheating of the feed must be performed in presence of oxygen because of high corrosion rates under oxygen-deficient conditions. To avoid oxygen-deficient conditions in the feed preheaters, the oxidizing gas must be added to the feed before the preheaters. This can be done only when the oxidizing gas is air and when the caustic waste does not contain easily oxidizable matter such as sulfides. For wastes containing sulfides, if air is added before the preheaters, a substantial portion of the oxidation would occur in the feed preheaters which will upset the energy balance in the wet oxidation reactor. On the other hand, if the oxidizing gas is pure oxygen, addition of the gas before the oil heater will result in unsafe operating conditions, with a risk of fire or explosion.
It has been observed that the severity of corrosion by the caustic wastes is reduced significantly when the pH is adjusted to below about 11.0, thereby permitting the wet oxidation components to be constructed of stainless steel which will result in substantial reduction in capital costs for the system. The feed pH can be lowered by adding an acid such as sulfuric acid. However, the operating cost for the treatment process will be high because of high chemical cost. Also, addition of sulfuric acid for pH adjustment will significantly increase the dissolved solids content of the waste which is undesirable.
The addition of various anti-corrosion chemical agents to liquids (e.g., boiler feedwater) is well known. Kitayama et al Japanese Patent Publication No. 52-16868 (Feb. 8, 1977) discloses the treatment of liquors which become highly acidic in the wet oxidation process and thus corrode the titanium equipment. Nitric acid, nitrous acid, chromic acid, or permanganic acid and one or more of the sodium, potassium or ammonium salts of those acids are added to the liquid prior to wet oxidation in order to form and/or maintain a protective oxidation film on the titanium equipment.